Liverpool's river (6)
I believe the answer is:
mersey
'liverpool's river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mersey that I've seen before include "River with ferry and tunnel" , "River flowing into Liverpool Bay" , "Route between Liverpool and the Wirral" , "Liverpool's river" , "River rising in Derbyshire and flowing into the Irish Sea" .)